Many modern day raster output scanners and raster input scanners utilize a rotating polygon having flat reflective surfaces or facets, in parallelism with the axis of rotation of the polygon. Rotational movement of such facets causes the reflected light of an incident, collimated beam to appear to revolve about an axis halfway between the center of rotation of the polygon and the facet. Such scanning systems often suffer from the effects of wobble, introduced into the system by inaccuracies in the bearings utilized to impart motion to the polygon and inaccuracies in the grinding and positioning of the actual facets on the faces of the polygon. The wobble effects cause an uneven raster scan which produces irregular locations of the output scan lines whether utilized in an input or an output fashion.
Cancellation of wobble by double reflection from the flat, parallel facets of a polygon, as proposed by U.S. Pat. No. 3,897,132, eliminates wobble but introduces excessive bow effects. U.S. patent application Ser. No. 532,323, filed Sept. 15, 1983, in the name of the same inventor and assignee of the present application, discloses a polygon scanning system for correcting wobble by double reflection from the active polygon facet, with the wobble correction being achieved without bow defects. Instead of the impinging light beam being transverse to the axis of rotation of a rotating polygon having facets in parallelism to the axis of rotation of the polygon, there is disclosed a rotating polygon with facets having a predetermined draft angle with the axis of rotation of the polygon, and a light beam impinging on the facets. Once again, by two further reflections from flat, fixed mirrors, the light beam is made to reimpinge upon the same facet; correcting wobble, but now with no effects of bow.
In such systems, after the second facet reflection the light beam is passed through conventional scan lens optics having appropriate power in the tangential plane, such that the scanning light beam is linearized and focused at the surface to be scanned. It would be advantageous to eliminate the scan lens optics, the advantage being not only a reduction in the component count, but also, if the function of the scan lens optics could be achieved by reflective surfaces or mirrors, the entire optical train would consist only of mirrors and thus have the same focus and scan characteristics at any wavelength.
Flat faceted polygons are made of metalized glass or solid metal. Such polygons cannot be made by injection molding using acrylic plastics, for example, because the surface tension of molten plastic is indeterminate for a flat surface and accordingly a facet cannot be made flat to sufficient accuracy. It would be desirable to have a polygon scanner that could be made by injection molding. Also, it would be beneficial if the injection molded polygon could achieve the function of the scan lens optics, the advantage being not only a reduction in the number of components, but also, if the function of the scan lens optics could be achieved by reflective surfaces, the entire optical plane could consist only of reflective surfaces and thus have the same focus and scan characteristics at all wavelengths, making the scanner usable with any color laser or light source.